Reactor for recovering phosphate salts from a liquid

ABSTRACT

The invention relates to a reactor ( 10 ) for completely separating phosphate from a liquid and for recovering phosphate salts, comprising a housing ( 12 ) and two electrodes having differing polarities, wherein a sacrificial anode ( 16 ) made of a magnesium-containing material and an inert cathode ( 18 ) are arranged concentrically inside the housing ( 12 ).

Reactor for recovering phosphate salts from a liquid

The invention concerns a reactor for complete separation of phosphatefrom a liquid and recovery of phosphate salts, comprising a housing andtwo electrodes of different polarity.

Phosphate salts such as magnesium ammonium phosphate (in the followingabbreviated as MAP) or potassium magnesium phosphate (in the followingabbreviated as PMP) are high-value plant adjuvants for which there ishigh demand. The elements nitrogen, potassium, magnesium, and phosphateof which these plant adjuvants are comprised are typically contained inall solid or liquid organic waste materials. While potassium, magnesiumand other ions are present in the form of water-soluble cations,nitrogen and phosphate are predominantly bound to or in organic materialor cell mass. Accordingly, a major proportion of nitrogen and phosphateare not available for the production of plant adjuvants. For this reasonit is necessary to convert nitrogen and phosphate into their inorganicform that is suitable for precipitation.

The spontaneous precipitation of MAP or PMP is limited by the usuallyvery low magnesium concentration in wastewater. Known is the addition ofmagnesium hydroxide, magnesium oxide or soluble magnesium salts for MAPprecipitation. The disadvantage in this context is the bad solubility ofthe oxides as well as of the salt-like hydroxides. Upon addition ofmagnesium hydroxide or magnesium oxide to the wastewater, thesecompounds dissolve only very slowly and with a minimal proportion. Thishas the result that it is necessary to continuously perform stirring ormixing which, however, causes extra expenditure in regard to technologyand energy and thus also with respect to costs. Moreover, bothcompounds, because of their bad solubility, must be added inover-stoichiometric amounts because otherwise an incompleteprecipitation of the desired plant adjuvants occurs and significantquantities of phosphate remain in the wastewater. When magnesium saltsare beforehand transferred into a solution, the efficiency of the methoddecreases because of the dilution with water.

The optimal pH value for precipitation of MAP is at 9. Wastewater hasusually pH values between 5 and 7. Therefore, for increasing the pHvalue, a base is added. By using a soluble base, for example, sodiumhydroxide. When using a base that is sparingly soluble, for example,magnesium hydroxide, the latter will hardly dissolve in water and theaforementioned disadvantages will occur.

A further possibility for adjusting a pH value that is favorable forprecipitation is disclosed in DE 101 12 934 B4. The aeration of primarysludge mentioned therein with subsequent CO₂ stripping is however veryenergy-intensive and causes therefore high additional costs.

WO 00200101019735 A1 discloses a reactor for removal of dissolvednitrogen and phosphate from the aqueous portion of liquid manure bymeans of electrochemical precipitation.

The reactor described therein requires relatively high electricalvoltages and is therefore energy-intensive and cost-intensive. Adisadvantage is also that nitrogen and phosphate that are presentorganically bound in the aqueous portion of the liquid manure cannot beremoved by the disclosed method. As a result of this, this wastewatermust therefore be subjected to a subsequent purification in a watertreatment plant. The construction of the reactor is disadvantageous inthat, due to the arrangement of the electrodes, large areas aregenerated in which the liquid to be purified has no direct contact withthe electrodes. Moreover, as a result of the geometry of the sacrificialanode more magnesium than required is released. Both effects reduce theefficiency of the reactor significantly. Moreover, in this method, dueto the use of aluminum-containing electrodes, the plant poison aluminumends up in the precipitation product. When this product is applied tothe soil, the aluminum can be released and the plant growth can beaffected negatively.

An electrochemical precipitation reactor for MAP is disclosed in WO2007/009749 A1. The reactor is however not suitable for theprecipitation of other phosphate salts. Also, the construction of thereactor does not allow for automatic separation between purifiedwastewater and precipitated MAP so that it is necessary to arrangeddownstream of the reactor a further apparatus for solid/liquidseparation. Also, a significantly higher apparatus expenditure isrequired because, as a result of the construction, the housing of thereactor cannot be operated as a cathode. In addition, the magnesium isdissolved non-uniformly because, due to the spatial arrangement of theelectrodes in the reactor, only one side of the anode is participatingin the reaction.

The invention has the object to provide a reactor by means of whichphosphate-containing wastewater can be treated and supplied to a furtheruse. Moreover, the invention has the object to provide a reactor forrecovery of phosphate salts as plant adjuvants which overcomes theaforementioned disadvantages of the prior art.

The object is solved according to the invention by a reactor with ahousing. At the center of the housing a sacrificial anode of magnesiumor a magnesium-containing material is arranged. An inert cathode isarranged concentrically about the sacrificial anode. In this way, thearrangement of the electrodes in accordance with the invention providesfor a best possible control of the release of magnesium ions. Since thespacing between magnesium anode and cathode is kept as minimal aspossible, the ions that are required for the precipitation of thephosphate salts are immediately in contact with each other and aconstantly high concentration of magnesium ions in the reaction space isensured. At the same time, because of the geometry of the sacrificialanode, its surface is very small so that only little magnesium isspontaneously released. In this way, advantageously an unnecessaryexcess of magnesium ions is prevented.

By means of the reactor according to the invention, it is possible byapplication of a minimal electrical direct current, smaller than 1 Vwith current strengths below 1 A, to supply magnesium ions to thephosphate-containing and ammonium-containing liquid and to split thewater that is contained in the liquid to OH⁻ and H⁺ ions so that the pHvalue is increased and the reactions required for precipitation can takeplace. Due to the minimal energy demand, the costs for operating thedevice drop in comparison to methods known from the prior art. It iseven possible to operate the reactor by galvanic operation. Electricalcurrent is produced thereby.

Moreover, it is proposed that an anaerobic fermentation process isarranged upstream of the reactor according to the invention. In thisfermentation process nitrogen and phosphorus that are bound organicallyare decomposed to inorganic water-soluble ions. From these ions,ammonium (NH₄ ⁺) and phosphate (PO₄ ³⁻), the phosphate salts, inparticular MAP and PMP, can be formed. In this way, nitrogen andphosphate that are bound predominantly to or in organic material or cellmass are converted advantageously into a water-soluble form and are thusavailable for the production of plant adjuvants. Moreover, in thisprocess biogas is produced which has a significant market value as anenergy source.

An advantageous embodiment of the invention provides that the housing ismanufactured of an electrically conductive material, for example, metaland therefore serves as an inert cathode. With the geometry of thereactor according to the invention and the concentric arrangement of thesacrificial anode the reaction space is limited to the space between thetwo electrodes. In this way, dead space is avoided and thus materialcosts are lowered.

A further advantage of the invention provides that the sacrificial anodeis comprised substantially of magnesium. Of course, this includes alsoelectrode materials that are comprised of a magnesium alloy or magnesiumwith minimal additions of other components.

In order to be able to advantageously perform the process of phosphatesalt recovery continuously, the reactor according to the presentinvention has an inlet for the phosphate-containing liquid, an outletfor the purified liquid, as well as a removal device for theprecipitated phosphate salts. The crystals can be removed via theremoval device by means of a shut-off valve, for example, a seat valveor disk valve or ball valve, from the reactor without the inlet oroutlet being changed and thereby the purification performance of thereactor being negatively affected.

An advantageous embodiment of the reactor according to the inventionprovides for the operation of the reactor as an upflow reactor. In thiscontext, the inlet is located laterally at the bottom end of thereactor. The wastewater flows upward and escapes laterally at the top.This arrangement has the advantage that an automatic separation ofliquid that flows upwardly and precipitated salts that sink to thebottom is taking place.

In principle, the reactor can be operated also as a downflow reactorwherein the liquid and solid move in the same direction. In this way,the sedimentation rate of the precipitated phosphate salts isaccelerated. This means that the reactor can be made smaller for thesame throughput.

In supplementing this, it is proposed that the crystals are separated ina filter from the liquid. In this way, in the reactor that is flowedthrough from top to bottom the precipitated phosphate salts can beremoved together with the liquid from the reactor. Accordingly,additional fixtures or devices for separate solids removal are saved.Also, in case of common removal of phosphate salts and purifiedwastewater, a turbulent flow is generated in the conduit and preventsclogging of the conduit by the crystals.

It is particularly beneficial when the housing of the reactor is closed.The electrolytic reactions that occur in the reactor produce largequantities of foam which in case of a closed housing cannot overflow.Accordingly, in an advantageous way, a product loss is avoided. Inprinciple, the housing of the reactor can also be open partially orentirely.

The reactor according to the invention operates even better when it hasa slanted bottom. In this way, it is possible that the precipitatedcrystals glide along the slanted surface downwardly and collect at theremoval device. In this way, the crystals can be removed from thereactor without its continuous operation having to be interrupted. Theformation of the slanted surface is realized by a preferably conicalbottom. Conceivable is also the shape of a pyramid.

An advantageous embodiment of the reactor according to the inventionprovides that to the sacrificial anode a positive pole and to thecathode a negative pole of a direct current source are connected. Thesupply of electrical current prevents deposits on the electrode whichare not stable in the electrical field. When the reactor, on the otherhand, is operated without a direct current source, we by the process ofmagnesium release electrons are released. This means that the reactorrequires no electrical current but even produces electrical current.

Further advantages and advantageous embodiments of the invention can betaken from the following Figures, their description, and the claims. Inthis context, all features disclosed in the Figures, their description,and the claims can be important for the invention individually as wellas in any combination with each other.

It is shown in:

FIG. 1 a schematic illustration of the reactor according to theinvention

FIG. 2 a schematic illustration of a first embodiment of the reactoraccording to the invention

FIG. 3 a schematic illustration of a second embodiment of the reactoraccording to the invention

FIG. 4 a schematic illustration of a third embodiment of the reactoraccording to the invention

FIG. 5 a schematic illustration of a third embodiment of the reactoraccording to the invention for recovery of phosphate salts and

FIG. 6 a schematic illustration of the use of the reactor according tothe invention with upstream fermentation process

FIG. 1 shows a schematic illustration of a reactor 10 according to theinvention. The reactor 10 has a housing 12. The housing 12 serves forreceiving a phosphate-containing liquid 14. At the center of the housing12 an electrode 16 is arranged.

The electrode 16 is a so-called sacrificial anode which is connected tothe positive pole of a direct current source, not illustrated in thedrawing, while the housing 12 forms the cathode 18 which is connected toa negative pole of the direct current source.

The sacrificial anode 16 is comprised of a magnesium-containing materialso that magnesium ions are transferred into the solution 14 as soon aselectrical voltage is applied to the electrodes 16 and 18.

Reaction equation for formation of MAP:

Mg²⁺+NH₄ ⁺+PO₄ ³⁻+6 H₂O→MgNH₄PO₄.6 H₂O

Reaction equation for formation of PMP:

Mg²⁺+K⁺+PO₄ ³⁻+6 H₂O→MgKPO₄.6 H₂O

Reaction equation for release of magnesium:

Mg(s)→Mg₂ ⁺+2e ⁻

Reaction equation for formation of hydroxide ions:

2 H₂O+2e ⁻→2 OH⁻+H₂

The formed phosphate salts are sparingly soluble in aqueous solution andprecipitate as crystals which deposit at a bottom of the reactor 10.

One configuration of the reactor 10 according to the invention providesfor galvanic operation. For this purpose, the two electrodes 16, 18 arenot connected to the external direct current source. The magnesium ionsare transferred into solution by galvanic operation.

FIG. 2 shows a first embodiment variant of the reactor 10 according tothe invention. Illustrated is the housing 12 of the reactor andcentrally arranged therein is the electrode 16 that is embodied as asacrificial anode. Concentric between housing 12 and electrode 16 thereis the cathode 18. In this special arrangement the spacing betweensacrificial anode 16 and inert cathode 18 is minimal so that the ionsthat are participating in the precipitation are immediately in contactwith each other.

In FIG. 3 the reactor 10 is illustrated. Laterally at a preferablyconical bottom 22 there is an inlet 24. An outlet 26 is located at thetop laterally at the housing 12 of the reactor 10. An optional returnline 28 connects the outlet 26 with the inlet 24. At the bottom end ofthe preferably conical bottom 22 there is the removal device 30.

The phosphate-containing liquid 14 flows through the inlet 24 from thebottom to the top through the reactor 10 and exits from it through theoutlet 26. The precipitated phosphate salts glide along the slantedplane of the conical bottom 22 in downward direction and are removed viathe removal device 30. In this way, the reactor 10 can be operatedcontinuously and the precipitated and deposited crystals can be removedat any time without changing a throughput of the reactor. By means ofthe optional return line 28, already purified liquid 14 is returned tothe reactor 10 as circulating water.

FIG. 4 shows a third embodiment of the reactor 10 according to theinvention. Here, the reactor 10 is flowed through in downward direction.The inlet 24 is located laterally at the top on the housing 12. Theoutlet 26 is located laterally at the conical bottom 22. The optionalreturn line 28 connects the outlet 26 with the inlet 24. At the conicalbottom 22 the removal device 30 is arranged.

The phosphate-containing liquid 14 flows through the inlet 24 from thetop to the bottom through the reactor 10 and exits from it through theoutlet 26. Precipitated phosphate salts are removed via the removaldevice 30 without affecting the operation of the reactor. By means ofreturn line 28 the already purified liquid 14 is supplied again to thereactor 10 as circulating water.

FIG. 5 shows a further embodiment of the reactor 10 according to theinvention. In this way, the reactor 10 is flowed through in downwarddirection. The inlet 24 is located laterally at the top at the housing12. The outlet 26 is located at the bottom end at the conical bottom 22and extends from there to a downstream filter 31. The optional returnline 28 connects the outlet 26 with the inlet 24.

In this fourth embodiment according to the invention, the precipitatedphosphate salts are removed together with the purified liquid 14 fromthe reactor 10. In the downstream filter 31 the phosphate salts areseparated from the liquid 14. In this context there is the possibilityto supply seed crystals to the reactor 10 via the return line 28.

In FIG. 6, an application of the reactor 10 according to the inventionin connection with producing biogas from phosphorus-containingwastewater is schematically illustrated.

A wastewater flow 32 of organic origin is supplied to a bioreactor 34.Herein, by anaerobic fermentation processes, the organic carboncompounds that are contained in the solids are converted into biogas andmineral residual substances. In this process, ammonium-containing andphosphate-containing process water 36 is produced. Before the processwater 36 is supplied through inlet 24 into the reactor 10, possiblycontained solids 40 are separated in a filter 38. The solids 40 whichare retained in the filter 38 are returned into the bioreactor 34. Inthe afore described way, in the reactor 10 according to the inventionthe phosphate salts are separated. The ammonium-containing andphosphate-containing outflow 26 can be returned partially into thebioreactor 34. In this way, an impairment of the fermentation process,caused by a high ammonium concentration, is prevented.

What is claimed is: 1.-11. (canceled)
 12. A reactor for completecrystallization and recovery of MAP (magnesium ammonium phosphate) andPMP (potassium magnesium phosphate) from a liquid and recovery ofphosphate salts, the reactor comprising: a housing; a first electrodeand a second electrode, wherein the first and second electrodes havedifferent polarity; wherein the first electrode is a sacrificial anodeof a magnesium-containing material; wherein the second electrode is aninert cathode; wherein the sacrificial anode and the inert cathode arearranged concentrically relative to each other; a return line connectedto the housing, wherein seed crystals are supplied to the reactor viathe return line.
 13. The reactor according to claim 11, wherein thehousing is comprised of an electrically conducting material and servesas the inert cathode.
 14. The reactor according to claim 11, wherein thesacrificial anode is comprised of magnesium.
 15. The reactor accordingto claim 11, further comprising an inlet, an outlet, and a removaldevice.
 16. The reactor according to claim 15, wherein the return linebranches off the outlet or branches of the removal device.
 17. Thereactor according to claim 11, that the reactor is flowed through invertical direction from the bottom to the top.
 18. The reactor accordingto claim 11, wherein the reactor is flowed through in vertical directionfrom the top to the bottom.
 19. The reactor according to claim 11,further comprising a filter that separates crystals of MAP, PMP, andphosphate salts from the liquid.
 20. The reactor according to claim 11,wherein the housing of the reactor is closed.
 21. The reactor accordingto claim 11, wherein the housing of the reactor is open.
 22. The reactoraccording to claim 11, comprising a slanted bottom.
 23. The reactoraccording to claim 11, comprising a direct current source, wherein apositive pole is connected to the sacrificial anode and a negative poleis connected to the inert cathode.